1. Field of the Invention
The present invention generally relates to the design of liquid lightguides, e.g. for use in medical endoscopy and in the curing of epoxy in dentistry and in industry, and more specifically to the selection of materials for tube, interior coating layer and for the transparent liquid.
2. Description of the Prior Art
Liquid lightguides with internal coating layers are already known since 1988.
In EP 0246 552 Fitz describes the possibility of coating internally flexible tubes made from thermoplastic or elastomeric material with an internally reflecting layer, made out of an amorphous fluoropolymer, called HOSTAFLON.RTM. TFB (Hoechst) or THV (3M Company), for use as a cladding of liquid core lightguides. The "highly amorphous" THV material from 3M can be dissolved in certain liquids, which allows for a simple coating process from solution by the way of dip coating.
In DE 423 3087 the present applicant describes a liquid core lightguide consisting of a tube made form a carbon fluoropolymer material, coated internally with a thin layer of TEFLON.RTM. AF, which is a new amorphous fluoropolymer, developed by the DuPont Company, comprising a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) with tetrafluorethylene (TFE).
Like THV from 3M, TEFLON.RTM. AF is also soluble in certain fluorinated liquids, thus allowing for a simple coating procedure via dip coating.
The advantages of internally coating plastic tubes, used for liquid lightguides with amorphous fluoropolymers are: Improved transmission due to the lack of crystallinity inside the reflective layer, and, especially when using TEFLON.RTM. AF, a higher numerical aperture of the lightguide, due to the extreme low value of the refractive index of TEFLON.RTM. AF, and as a consequence of the increased aperture, less dependence of the transmission on the bending radius of the tube.
The internal coating of tubes with TEFLON.RTM. AF, however, has also some disadvantages: DuPont, being the sole producer of TEFLON .RTM. AF, charges 10 US$ per gram of the material. This means already considerable cost per meter of liquid lightguide, even with the small thickness of the applied layers being in the range of a few microns.
Furthermore, the adhesion of thin layers of TEFLON.RTM. AF applied from solution to the surface of plastic materials other than fluorocarbon polymers is still insufficient.
Especially the baking procedure at temperatures up to 250.degree. C., required by DuPont to achieve good adhesion to the plastic surface, does not leave much of a choice for others than fluorocarbon polymers as a substrate material, being one of the few plastic materials, that can be heated up to such high temperatures.
The adhesion of TEFLON.RTM. AF to substrate materials of glass is also insufficient.